13.2 Fugitive Dust Sources

Significant atmospheric dust arises from the mechanical disturbance of granular material

exposed to the air. Dust generated from these open sources is termed "fugitive" because it is not

discharged to the atmosphere in a confined flow stream. Common sources of fugitive dust include

unpaved roads, agricultural tilling operations, aggregate storage piles, and heavy construction

operations.

For the above sources of fugitive dust, the dust-generation process is caused by 2 basic

physical phenomena:

1. Pulverization and abrasion of surface materials by application of mechanical force

through implements (wheels, blades, etc.).

2. Entrainment of dust particles by the action of turbulent air currents, such as wind erosion

of an exposed surface by wind speeds over 19 kilometers per hour (km/hr) (12 miles per

hour [mph]).

In this section of AP-42, the principal pollutant of interest is PM-10 — particulate matter (PM)

no greater than 10 micrometers in aerodynamic diameter (μmA). Because PM-10 is the size basis for

the current primary National Ambient Air Quality Standards (NAAQS) for particulate matter, it

represents the particle size range of the greatest regulatory interest. Because formal establishment of

PM-10 as the primary standard basis occurred in 1987, many earlier emission tests have been

referenced to other particle size ranges, such as:

TSP Total Suspended Particulate, as measured by the standard high-volume ("hi-vol") air

sampler, has a relatively coarse size range. TSP was the basis for the previous

primary NAAQS for PM and is still the basis of the secondary standard. Wind tunnel

studies show that the particle mass capture efficiency curve for the high-volume

sampler is very broad, extending from 100 percent capture of particles smaller than

10 μm to a few percent capture of particles as large as 100 μm. Also, the capture

efficiency curve varies with wind speed and wind direction, relative to roof ridge

orientation. Thus, high-volume samplers do not provide definitive particle size

information for emission factors. However, an effective cut point of 30 μm

aerodynamic diameter is frequently assigned to the standard high volume sampler.

SP Suspended Particulate, which is often used as a surrogate for TSP, is defined as PM

with an aerodynamic diameter no greater than 30 μm. SP may also be denoted as PM30.

IP Inhalable Particulate is defined as PM with an aerodynamic diameter no greater than

15 μm IP also may be denoted as PM-15.

FP Fine Particulate is defined as PM with an aerodynamic diameter no greater than

2.5 μm. FP may also be denoted as PM-2.5.

The impact of a fugitive dust source on air pollution depends on the quantity and drift

potential of the dust particles injected into the atmosphere. In addition to large dust particles that

1/95 Miscellaneous Sources 13.2-1

AP-42 Compilation of Air Pollutant Emission Factors, Chapter 13 , Introduction to Fugitive Dust Sources. January 1995.

USEPA, Office of Air Quality Planning and Standards. http://www.epa.gov/ttn/chief/ap42/ch13/final/c13s02.pdf
settle out near the source (often creating a local nuisance problem), considerable amounts of fine

particles also are emitted and dispersed over much greater distances from the source. PM-10

represents a relatively fine particle size range and, as such, is not overly susceptible to gravitational

settling.

The potential drift distance of particles is governed by the initial injection height of the

particle, the terminal settling velocity of the particle, and the degree of atmospheric turbulence.

Theoretical drift distance, as a function of particle diameter and mean wind speed, has been computed

for fugitive dust emissions. Results indicate that, for a typical mean wind speed of 16 km/hr

(10 mph), particles larger than about 100 μm are likely to settle out within 6 to 9 meters (20 to 30 feet

[ft]) from the edge of the road or other point of emission. Particles that are 30 to 100 μm in diameter

are likely to undergo impeded settling. These particles, depending upon the extent of atmospheric

turbulence, are likely to settle within a few hundred feet from the road. Smaller particles, particularly

IP, PM-10, and FP, have much slower gravitational settling velocities and are much more likely to

have their settling rate retarded by atmospheric turbulence.

Control techniques for fugitive dust sources generally involve watering, chemical stabilization,

or reduction of surface wind speed with windbreaks or source enclosures. Watering, the most common

and, generally, least expensive method, provides only temporary dust control. The use of chemicals to

treat exposed surfaces provides longer dust suppression, but may be costly, have adverse effects on

plant and animal life, or contaminate the treated material. Windbreaks and source enclosures are often

impractical because of the size of fugitive dust sources.

The reduction of source extent and the incorporation of process modifications or adjusted work

practices, both of which reduce the amount of dust generation, are preventive techniques for the

control of fugitive dust emissions. These techniques could include, for example, the elimination of

mud/dirt carryout on paved roads at construction sites. On the other hand, mitigative measures entail

the periodic removal of dust-producing material. Examples of mitigative control measures include

clean-up of spillage on paved or unpaved travel surfaces and clean-up of material spillage at conveyor

transfer points.

13.2-2 EMISSION FACTORS 1/95

AP-42 Compilation of Air Pollutant Emission Factors, Chapter 13 , Introduction to Fugitive Dust Sources. January 1995.

USEPA, Office of Air Quality Planning and Standards. http://www.epa.gov/ttn/chief/ap42/ch13/final/c13s02.pdf